The present invention relates generally to coating compositions and, more particularly, to electrodepositable coating compositions providing improved chip resistance and edge coverage and/or reduced cratering.
Electrodepositable coatings have become the coatings of choice for corrosion protection for metal substrates, such as those used in the automobile industry. Electrodeposition processes provide higher paint utilization, outstanding corrosion protection, low environmental contamination, and a highly automated process compared to conventional non-electrophoretic coating methods.
The electrodeposition process involves immersing an electroconductive substrate into a bath of an aqueous electrocoating composition, the substrate serving as a charged electrode in an electrical circuit comprising the electrode and an oppositely charged counter-electrode. Sufficient electrical current is applied between the electrodes to deposit a substantially continuous, adherent film of the electrocoating composition onto the surface of the electroconductive substrate.
U.S. Pat. Nos. 4,148,772 and 4,104,147 disclose electrocoating compositions including polyepoxides chain extended with polyester polyols. Reaction occurs by the alcoholic primary hydroxyl of the polyester polyol opening the epoxide ring forming an ether linkage and a secondary hydroxyl group (U.S. Pat. No. 4,148,772 at column 3, lines 33-35). The chain extended polyepoxides are reacted with a secondary amine and made dispersible in water with acid.
U.S. Pat. No. 4,536,558 discloses electrodepositable resins which are prepared by reacting polyepoxides with polycarboxylic acids to produce chain extended polyepoxides. The chain extended polyepoxides are then reacted with secondary amines or, optionally, with blends of secondary amines with minor amounts of primary amines, care being taken to avoid gelation.
A problem associated with conventional electrodeposition processes is poor coating coverage at the edges of the coated substrate. The coating thickness at the substrate edges tends to be relatively thinner than that of the non-edge portions of the substrate, which can result in the early onset of corrosion at the substrate edges. Additionally, poor flow characteristics of the coating composition may result in pits or craters formed in the coating.
U.S. Pat. No. 5,096,556 discloses a cationic polyepoxide-amine reaction product and a polyepoxide crosslinking agent which are dispersed in water and heated to crosslink to form a cationic microgel dispersion. The dispersion can be combined with ungelled cationic resins to form an aqueous resinous dispersion suitable for electrodeposition which enhances edge coverage.
An object of the present invention is to provide a composition and process for the production of coatings with improved edge coverage and chip resistance and reduced cratering tendency while additionally providing efficiencies in application and processing, such as improved flow control and/or improved solvent resistance.
The present invention provides a curable coating composition which comprises an at least partially gelled reaction product prepared from reactants comprising: (a) an epoxy functional material and (b) a polyoxyalkylene polyamine.
Another aspect of the present invention is a curable coating composition comprising an at least partially gelled ionic reaction product prepared from reactants comprising: (a) an epoxy functional polyester and (b) an amine-functional material.
A curable coating composition is provided which comprises an at least partially gelled ionic reaction product prepared from reactants comprising an epoxy functional polyester and a primary amine, wherein the reactants are essentially free of secondary amines.
A coated substrate of the invention has on at least a portion of a surface thereof a coating deposited from a coating composition which comprises: (A) a curable, at least partially gelled reaction product prepared from reactants comprising (i) an epoxy functional polyester; and (ii) a polyoxyalkylene polyamine; and (B) a curing agent.
Another coated substrate of the present invention has on at least a portion of a surface thereof a coating deposited from a coating composition which comprises: (A) an at least partially gelled ionic reaction product prepared from reactants comprising: (i) an epoxy functional polyester prepared from reactants comprising: (1) the reaction product of a polyol and a polycarboxylic acid or anhydrides thereof; and (2) an epoxy functional material; and (ii) an amine-functional material; and (B) a curing agent.
Another aspect of the present invention is a coated substrate having on at least a portion of a surface thereof a coating deposited from a coating composition which comprises an at least partially gelled ionic reaction product prepared from reactants comprising an epoxy functional polyester and a primary amine, wherein the reactants are essentially free of secondary amines.
A method of electrocoating an electrically conductive surface of the present invention comprises passing electric current between an anode and a cathode to cause an electrodepositable coating composition to deposit upon the electrically conductive surface, wherein the coating composition comprises an at least partially gelled reaction product prepared from reactants comprising: (i) an epoxy functional polyester; and (ii) a polyoxyalkylene polyamine.
Another method of electrocoating an electrically conductive surface according to the present invention comprises passing electric current between an anode and a cathode to cause an electrodepositable coating composition to deposit upon the electrically conductive surface, wherein the coating composition comprises an at least partially gelled ionic reaction product prepared from reactants comprising: (i) an epoxy functional polyester prepared from reactants comprising (1) the reaction product of a polyol and a polycarboxylic acid or anhydrides thereof, and (2) an epoxy functional material; and (ii) an amine-functional material.
Yet another method of electrocoating an electrically conductive surface according to the present invention comprises passing electric current between an anode and a cathode to cause an electrodepositable coating composition to deposit upon the electrically conductive surface, wherein the coating composition comprises an at least partially gelled ionic reaction product prepared from reactants comprising an epoxy functional polyester and a primary amine, and wherein the reactants are essentially free of secondary amines.
Yet another method of electrocoating an electrically conductive surface according to the present invention comprises passing electric current between an anode and a cathode, and sequentially depositing first and second electrodepositable coating compositions to form a first coating and a second coating on the electrically conductive surface, wherein the first coating composition comprises an electrically conductive material, and wherein the second coating composition comprises an at least partially gelled reaction product prepared from reactants comprising: (i) an epoxy functional polyester; and (ii) a polyoxyalkylene polyamine.
A still further method of coating a substrate comprises providing a substrate and applying a coating composition upon at least a portion of the substrate, the coating composition comprising an at least partially gelled reaction product prepared from reactants comprising an epoxy functional polyester and a polyoxyalkylene polyamine.
Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term xe2x80x9caboutxe2x80x9d. Also, as used herein, the term xe2x80x9cpolymerxe2x80x9d is meant to refer to oligomers and both homopolymers and copolymers. Additionally, any numeric reference to amounts, unless otherwise specified, are xe2x80x9cby weightxe2x80x9d; for instance, the phrase xe2x80x9csolids of 34%xe2x80x9d means xe2x80x9csolids of 34% by weightxe2x80x9d.
The curable coating compositions of the invention comprise one or more at least partially gelled reaction products or additives. The at least partially gelled, e.g., ionic, reaction products of the invention are prepared by reacting one or more epoxy functional polyesters with one or more amine functional materials. The reaction products may be used alone or in combination with another resin and/or other additives to provide curable coating compositions of the present invention. In the following discussion, the coating compositions of the invention are discussed in the context of electrodepositable coating compositions. However, it is to be understood that the coating compositions of the invention are not limited to electrodepositable coatings but can also be used in other coating processes, such as spray coating processes for primers, basecoats, clearcoats or refinish coating compositions.
Epoxy functional polyesters suitable for use in the curable coating composition of the present invention can be prepared by reacting (1) the reaction product of one or more polyols and one or more polycarboxylic acids or anhydrides thereof, and (2) one or more epoxy functional materials.
Examples of suitable polyols useful for forming the polyester reaction product (1) include resorcinol, dihydroxy benzene, aliphatic, cycloaliphatic or aralaphatic hydroxyl containing compounds, such as ethylene glycol, propylene glycol, hydrogenated bisphenol A, dihydroxyl cyclohexane, dimethylol cyclohexane, or combinations thereof. In a preferred embodiment, the polyol is saturated or hydrogenated bisphenol A. The polyol is present in an amount of about 10 weight percent to about 70 weight percent, preferably about 20 to about 50 weight percent, of the reactants to form the reaction product (1).
Examples of suitable polycarboxylic acids and anhydrides thereof useful for forming reaction product (1) for the practice of the invention include aromatic or aliphatic polycarboxylic acids, such as succinic acid, adipic acid, azelaic acid, sebacic acid, maleic acid, fumeric acid, phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, trimellitic acid and cyclohexane dicarboxylic acid and anhydrides thereof. In a preferred embodiment, the anhydride of the polycarboxylic acid is methyl hexahydrophthalic anhydride. The acid or anhydride comprises about 30 weight percent to about 90 weight percent, preferably about 50 weight percent to about 80 weight percent, of the reactants. The polyol and acid or anhydride can be reacted with or without a catalyst in any conventional manner known in the art to form the polyester reaction product (1).
The polyester reaction product (1) is reacted with one or more epoxy functional materials. Suitable epoxy-functional materials for the practice of the invention preferably contain at least one epoxy or oxirane group in the molecule, such as di- or polyglycidyl ethers of polyhydric alcohols. Preferably, the epoxy-functional material contains one epoxy group per molecule. Preferred epoxy functional materials are epihalohydrins, such as epichlorohydrin.
Other useful epoxy functional materials include, for example, polyglycidyl ethers of polyhydric alcohols which can be formed by reacting epihalohydrins, such as epichlorohydrin, with polyhydric alcohols, such as dihydric alcohols, in the presence of an alkali condensation and dehydrohalogenation catalyst such as sodium hydroxide or potassium hydroxide. Suitable polyhydric alcohols can be aromatic, aliphatic or cycloaliphatic. An example of a useful saturated polyglycidyl ether of polyhydric alcohol is EPONEX 1510 which is commercially available from Shell Chemical Company of Houston, Tex.
Suitable epoxy-functional materials preferably have an epoxy equivalent weight ranging from about 90 to about 2000, as measured by titration with perchloric acid using methyl violet as an indicator. About 5 weight percent to about 70 weight percent and preferably about 10 to about 25 weight percent of the epoxy functional material is combined or reacted with the polyester described above to form the reaction product of the invention.
Preferably, the epoxy functional polyester is free or essentially free of hydroxy functionality. The term xe2x80x9cessentially freexe2x80x9d of hydroxy functionality means that the epoxy functional polyester contains less than about 10% hydroxy functionality, preferably less than about 5%, and most preferably is free of hydroxy functionality.
An example of a suitable epoxy functional polyester of the present invention can be produced by first esterifying saturated or hydrogenated bisphenol-A with about two equivalents of 4-methyl hexahydrophthalic anhydride, and reacting the resulting polyester with about two equivalents of epichlorohydrin. Preferably, this polyester has the following general formula I: 
where n ranges from about 1 to about 15 or, preferably, from about 1 to about 5. As will be appreciated by one of ordinary skill in the art, when preparing the above polyester with a desired n value, some polyesters of different n values can be formed yielding a blended material primarily comprising the desired polyester but also including other similar polyester reaction products.
Another example of a suitable epoxy functional polyester can be prepared by esterification of 2-ethyl-2-butyl-1,3-propane diol with about two equivalents of 4-methyl hexahydrophthalic anhydride, then reacting the resulting polyester with about two equivalents of epichlorohydrin. The alternative epoxy functional polyester comprises a branched aliphatic material having the following general formula II: 
where n ranges from about 1 to about 15, or, preferably, ranging from about 1 to about 5. Rxe2x80x2 and Rxe2x80x3 represent independently selected hydrogen, alkyl groups or aromatic groups. The alkyl groups can be, for example, methyl, ethyl, propyl or butyl groups. Preferably, Rxe2x80x2 is an ethyl group and Rxe2x80x3 is a butyl group.
Another example of a suitable epoxy functional polyester can be prepared by reacting saturated bisphenol A and about two equivalents of 4-methyl hexahydrophthalic anhydride and reacting the resulting polyester with a saturated polyglycidyl ether of a polyhydric alcohol, such as EPONEX 1510.
In the practice of the invention, the epoxy functional polyester is reacted with one or more amine functional materials, such as primary, secondary, or tertiary amines, or polymers thereof. Preferred amine functional materials useful in the practice of the invention include primary amines, such as butyl amine or hexyl amine, more preferably diamines, such as ethyl diamine, 1,4-butanediamine or 1,6-hexanediamine.
In a preferred practice of the invention, the amine functional material preferably comprises polyoxyalkylene polyamines, such as polyoxymethylene diamine, polyoxyethylene diamine, polyoxypropylene diamine, polyoxybutylene diamine or mixtures thereof.
An example of a preferred polyoxyalkylene polyamine is represented by the following structural formula: 
where m can range from 0 to about 50, n can range from about 1 to about 50, nxe2x80x2 can range from about 1 to about 50, y can range from 0 to about 50, and R1 through R6 can be the same or different and can be independently selected from the group consisting of hydrogen or lower alkyl radicals preferably having about 1 to about 6 carbon atoms.
Another example of a preferred polyoxyalkylene polyamine is represented by the formula: 
where R, Rxe2x80x2 and Rxe2x80x3 can be the same or different and are independently selected from the group consisting of hydrogen or lower alkyl radicals preferably having from about 1 to about 6 carbon atoms. The number m preferably ranges from 0 to about 6 and the number n can range from about 1 to about 50, preferably from about 4 to about 40.
Non-limiting examples of preferred polyoxyalkylene amines include Jeffamine(copyright) D-2000 and Jeffamine(copyright) D-400 which are commercially available from Huntsman. Huntsman Technical Bulletin for Jeffamine D-2000 Polyoxypropylene Diamine, 1994, hereby incorporated by reference, describes a preferred polyoxypropylene diamine and gives an average value of 33.1 for n. Particularly preferred is the compound in which R, Rxe2x80x2 and Rxe2x80x3 are all methyl radicals. Other useful polyoxyalkylene polyamines are described in more detail in U.S. Pat. No. 3,236,895, column 2, lines 40-73; methods of preparation of the polyoxyalkylene polyamines are illustrated in the patent in Examples 4, 5, 6 and 8-12 in columns 4-9 thereof; the aforementioned portions of U.S. Pat. No. 3,263,895 hereby being incorporated by reference.
It is preferred, for the preparation of a reaction product of the present invention with the desired characteristics, that the amine functional material used in the practice of the invention be essentially free of secondary amines. As used herein, xe2x80x9cessentially free of secondary aminesxe2x80x9d means that the reactants from which the reaction product of the epoxy functional polyester and amine functional material are prepared comprises less than about 10% by weight, preferably less than about 5%, and more preferably is free of secondary amines.
In a currently preferred method of preparing the reaction products of the invention, a prepolymer material is first prepared. To form the prepolymer, the epoxy functional polyester, for example 80% solids by weight dissolved in methyl isobutyl ketone, is added to the amine functional material and heated to a temperature of about 50xc2x0 to 120xc2x0 C., preferably from about 80xc2x0 C. to about 110xc2x0 C., for about 11 hours to form a non-gelled prepolymer. The reaction is preferably allowed to proceed until about 30% to about 80% of the epoxy groups are reacted. The prepolymer material is then dispersed in aqueous medium and at least partially neutralized with acid. Suitable acids include organic acids such as formic acid, lactic acid and acetic acid, and inorganic acids such as sulfamic acid. For example, the prepolymer material can be added to a premix of sulfamic acid in water, with the sulfamic acid present in an amount of about 0.2 to about 0.9 equivalents of acid per one equivalent of amine to form a dispersion. This dispersion is then heated to a temperature sufficient to advance crosslinking or gellation of the epoxy polyester and amine-functional material, e.g. about 50xc2x0 C. to about 98xc2x0 C. for about 1 to about 10 hours, to form a cationic dispersion of an at least partially gelled reaction product of the invention.
The term xe2x80x9cdispersionxe2x80x9d as used within the context of the present invention means a two-phase transparent, translucent or opaque aqueous resinous system in which the reaction product or resin is the dispersed phase and water the continuous phase.
By xe2x80x9cat least partially gelledxe2x80x9d is meant that the reaction product of the epoxy polyester and the amine-functional material is at least partially crosslinked. The degree of crosslinking or xe2x80x9cgellingxe2x80x9d of the reaction product can be determined in conventional manner, for example, by recovering crosslinked microparticles from the dispersion, such as by removing the diluent by evaporation. The dried material can be placed in a Soxhlet extractor and extracted for about 12 hours in boiling solvent, such as tetrahydrofuran. To be considered xe2x80x9cgelledxe2x80x9d, at least 25 percent by weight, preferably 50 percent by weight, of the microparticles should not be extractable. A suitable such procedure is disclosed in U.S. Pat. No. 5,096,556, at column 2, lines 39-46.
Another way of determining whether the reaction product of the invention is at least partially gelled is to take about 1.0 ml to about 10.0 ml of the dispersion containing the reaction product of the epoxy polyester and amine-functional material (about 30 weight percent solids in water) and heating the reaction product to a temperature of about 95xc2x0 C. After two hours of heating, a one gram sample of the dispersion is taken each hour and added to about 10 grams of a solvent known not to attack the polymer, such as DMSO, acetone, or tetrahydrofuran. Gelling is indicated when the sample exhibits visual haziness.
The dispersion can include solvents such as hydrocarbons, ethers, alcohols, ether-alcohols and ketones, such as methyl isobutyl ketone. The amount of solvent used can vary from about 0 to about 90 weight percent, preferably about 5 to about 50 weight percent based on total weight of the reaction mixture.
The equivalent ratio of epoxy groups in the epoxy functional material (i) to amine groups in which the amine functional material (ii) is a primary diamine generally can range from about 1.1:1 to about 3.9:1 to produce reaction products having desirable properties. Equivalent ratios greater than about 3.9:1 and less than about 1.1:1 generally are not preferred because a gel may not be formed. The above discussed equivalent ratios relate epoxy groups to amine groups. As will be appreciated by one of ordinary skill in the art, these ratios would be different if other standards were used as the basis of the ratio, such as relating NH groups, for example, to epoxy groups. For amine-functional materials generally, the ratio of xe2x80x94NH equivalents to epoxy equivalents can range from about 0.51:1 to about 1.81:1.
The reaction product of the epoxy functional polyester and the amine functional material is characterized as being substantially free from primary hydroxyl functionality and includes epoxy functionality, preferably secondary or tertiary amine functionality with little or no primary amine functionality.
Preferred coating compositions of the invention comprise a percentage by weight of the at least partially gelled reaction product within the range from about 0.1% to about 50% on a basis of total resin solids, preferably about 1 to about 10 weight percent. The concentration of the resinous phase in aqueous medium depends upon the particular end use of the dispersion and is generally not critical. The reaction products of the invention can be dispersed in a carrier, such as water and/or solvent. The aqueous medium and the dispersion can contain from at least about 0.01, and usually from about 0.01 to about 20 percent by weight of the reaction product of the invention based on total weight of the aqueous composition.
The at least partially gelled reaction product of the invention can preferably be used in cationic electrodepositable coating compositions or can be modified to form an anionic coating composition. The system will require different chemistry for anionic systems.
The electrodepositable compositions of the invention preferably further comprise one or more cationic crosslinkable or self-crosslinking film-forming materials or resins such as epoxy resins, polyester resins different from the reaction product described above, polyurethane resins, acrylic resins, copolymers and mixtures thereof in an amount of about 20 to about 99.9 weight percent.
Suitable epoxy functional resins for use as film-forming resins include epoxy functional materials discussed above which are different from the epoxy functional polyester, for example, those disclosed in U.S. application Ser. No. 09/309,850 and U.S. Pat. No. 5,820,987, herein incorporated by reference.
Examples of suitable polyester film forming resins are disclosed, for example, in U.S. Pat. Nos. 5,739,213 and 5,811,198 and the concurrently filed application entitled xe2x80x9cCationic Aliphatic Polyester Resins and Their Use in Electrodepositionxe2x80x9d, herein incorporated by reference. The polyester film-forming resins have different chemical structures than the epoxy polyester reaction product discussed above. An exemplary polyester film-forming material comprises the reaction product of an aromatic and/or cycloaliphatic carboxylic acid compound comprising at least two aromatic and/or secondary aliphatic carboxyl groups, or an anhydride thereof; a branched aliphatic, cycloaliphatic or araliphatic compound containing at least two aliphatic hydroxyl groups, the aliphatic hydroxyl groups being either secondary or tertiary hydroxyl groups or primary hydroxyl groups attached to a carbon adjacent to a tertiary or quaternary carbon; a compound comprising an ionic salt group or a group which is converted to an ionic salt group; and optionally, at least one hydroxyl substituted carboxylic compound comprising at least one tertiary aliphatic carboxyl group and at least two aliphatic hydroxyl groups. Preferably, the ionic salt group equivalent weight of the polyester polymer is between 1,000 and 10,000.
Useful polyurethane resins include the reaction products of polymeric polyols such as polyester polyols or acrylic polyols with a polyisocyanate, including aromatic diisocyanates such as 4,4xe2x80x2-diphenylmethane diisocyanate, aliphatic diisocyanates such as 1,6-hexamethylene diisocyanate, and cycloaliphatic diisocyanates such as isophorone diisocyanate and 4,4xe2x80x2-methylene-bis(cyclohexyl isocyanate).
Suitable acrylic-functional resins include polymers derived from alkyl esters of acrylic acid and methacrylic acid such as are disclosed, for example, in U.S. Pat. Nos. 3,455,806 and 3,928,157, which are incorporated herein by reference.
The coating composition of the invention preferably further comprises one or more curing or crosslinking agents for curing curable groups of the at least partially gelled reaction product and/or the film-forming material. Suitable crosslinking agents include aminoplast resins, blocked or unblocked polyisocyanates, and mixtures thereof. For electrodepositable coating compositions, blocked polyisocyanates are preferred. Instead of a separate curing agent, the film-forming resin may be self-crosslinkable, i.e., may include crosslinkable components as part of the resin.
Useful curing agents include blocked or unblocked polyisocyanates, including aromatic diisocyanates such as p-phenylene diisocyanate, 4,4xe2x80x2-diphenylmethane diisocyanate and 2,4- or 2,6-toluene diisocyanate; aliphatic diisocyanates such as 1,4-tetramethylene diisocyanate and 1,6-hexamethylene diisocyanate; and cycloaliphatic diisocyanates such as isophorone diisocyanate and 4,4xe2x80x2-methylene-bis(cyclohexyl isocyanate). Examples of suitable blocking agents for the polyisocyanates include amines, such as dibutyl amine, lower aliphatic alcohols such as methanol, oximes such as methyl ethyl ketoxime amines and lactams such as caprolactam. The amount of the curing agent in the electrodepositable coating composition can range from about 5 to about 50 weight percent on a basis of total resin solids weight of the electrodepositable coating composition.
Other useful aminoplast resins are based on the condensation products of formaldehyde, with an amino- or amido-group carrying substance. Condensation products obtained from the reaction of alcohols and formaldehyde with melamine, urea or benzoguanamine are most common and preferred herein. However, condensation products of other amines and amides can also be employed, for example, aldehyde condensates of triazines, such as diazines, triazoles, guanidines, guanamines and alkyl and aryl-substituted derivatives of such compounds.
The electrodepositable coating composition of the invention also can comprise one or more pigments which can be incorporated in the form of a paste, surfactants, wetting agents, catalysts, film build additives, flatting agents, defoamers, flow control agents, pH control additives and carrier materials such as water and/or organic solvents. Useful pigments and fillers for the practice of the invention include iron oxides, lead oxides, carbon black, titanium dioxide, clay and talc.
In a currently preferred practice, the coating composition of the invention can be used to form a single layer, i.e. monocoat, coating on a substrate. Alternatively, the coating composition of the invention can be used to form one or more of multiple layers of coatings upon the substrate, which can be pretreated as described below. Still further, the at least partially gelled reaction product of the invention can be used as an additive in a conventional coating composition to improve flow characteristics and promote edge coverage. For example, the reaction product can be present in an electrodepositable coating composition as the second coat of a two-coat electrodeposited coating. The first coat of such a two-coat system preferably comprises an electrically conductive coating material comprising a curable film-forming resin as discussed above, a curing agent and an electrically conductive pigment.
The first coat is formed by electrodepositing the electrically conductive coating material onto the surface of an electrically conductive substrate. The first coating composition may be an anionic electrodepositable composition or a cationic electrodepositable composition, which is preferred. Preferred anionic and cationic electrodepositable compositions are those which provide high throwpower and good corrosion resistance. Suitable first coating compositions are well known in the art and are disclosed, for example, in U.S. patent application Ser. No. 09/309,851, herein incorporated by reference.
Examples of ionic resins suitable for use in such cationic electrodepositable coating compositions include amine salt group-containing resins such as the acid-solubilized reaction products of polyepoxides and primary or secondary amines which can be used in combination with a partially or fully blocked isocyanate curing agent. Other useful ionic resins and components of electrodepositable coating compositions are described in U.S. patent application Ser. No. 09/309,851.
The electrodepositable compositions for the first coat contain an electroconductive pigment to make the resultant coating electroconductive. Suitable electroconductive pigments include electrically conductive carbon black pigments, such as described in U.S. application Ser. No. 09/309,851.
The method of applying the first coating composition makes use of the electrodepositable compositions described above and depends upon the electrical conductivity of the substrate material. Most electroconductive substrates, especially metal substrates such as steel, zinc, aluminum, copper, magnesium, or the like and galvanized metals can be coated with the electrodepositable compositions. In a currently preferred practice, the substrate is steel or galvanized steel. Prior to treatment according to the method of the present invention, the metal substrate can be cleaned and degreased and a pretreatment coating, such as CHEMFOS 700 zinc phosphate or BONAZINC zinc-rich pretreatment (each commercially available from PPG Industries, Inc. of Pittsburgh, Pa.), can be deposited upon the surface of the metal substrate.
In a preferred method of applying the electrically conductive first coat, the aqueous dispersion of the first electrodepositable composition is placed in contact with an electrically conductive anode and cathode. Upon passage of an electric current between the anode and cathode, an adherent film of the electrodepositable composition will deposit in a substantially continuous manner on either the anode or the cathode depending on whether the composition is anionically or cationically electrodepositable. Electrodeposition is usually carried out at a constant voltage in the range of from about 1 volt to several thousand volts, typically between 50 and 500 volts. Current density is usually between about 1.0 ampere and 15 amperes per square foot (10.8 to 161.5 amperes per square meter).
After electrodeposition, the first coat is at least partially cured, typically by heating in air. Temperatures usually range from 200xc2x0 F. to 400xc2x0 F. (93.3xc2x0 C. to 204.4xc2x0 C.), preferably from 300xc2x0 F. to 375xc2x0 F. (149xc2x0 C. to 191xc2x0 C.) for a period of time ranging from 10 to 60 minutes. The thickness of the resultant film is usually from about 10 to 50 microns.
The heating or baking of the electrodeposited first coat can also be performed by means of infrared radiation (xe2x80x9cIRxe2x80x9d). Curing can be done in a selected manner, such as described in U.S. application Ser. No. 09/309,851.
After application of the first electroconductive coating, the second coating composition preferably is electrodeposited over the first coat. The second coating composition comprises the reaction product of an epoxy functional polyester and an amine, preferably a polyoxyalkylene polyamine, as described above, a curing agent and optionally pigment, etc. as described above. The second electrodeposited coating can be cured in a similar manner to that described above for the first coating composition. In a currently preferred practice, the preferred film former is a self-curing polyurethane as set forth in Table I below.
The combination of coatings not only provides corrosion resistance, but can provide enhanced chip and humidity resistance. The first electrodeposited coat can provide corrosion resistance and the second electrodeposited coat can enhance chip resistance and humidity resistance as well as the workability of the system.
Illustrating the invention are the following examples which, however, are not to be considered as limiting the invention to their details. All parts and percentages in the following examples as well as throughout the specification are by weight unless otherwise indicated.